WO2013155453A1 - Carbon nanotube membrane with tunable properties - Google Patents

Abstract

In one aspect, the invention relates to semi-permeable filtration membranes comprising carbon nanotube-templated conjugated polymer nanofiber composites for use in, for example, cross-flow electro-filtration applications, and methods for making and using same. Also disclosed are methods for compatibilizing carbon nanotubes, methods for modifying the conductivity of carbon nanotubes, and methods for tuning flux in a semipermeable filtration membrane. This abstract is intended as a scanning tool for purposes of searching in the particular art and is not intended to be limiting of the present invention.

Description

CARBON NANOTUBE MEMBRANE WITH TUNABLE PROPERTIES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the benefit of U.S. Provisional Application No.

61/623,531, filed on April 12, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Microfiltration (MF) and ultrafiltration (UF) membranes include flat sheet and hollow fiber thin films generally formed from polymeric materials and having a substantially continuous matrix structure containing open pores or conduits of small size. The pore size range for pores of MF/UF membranes are generally understood to extend from about 0.003 microns to 10's of microns. Polymeric MF/UF membranes are commonly prepared by the phase inversion method in which a polymer solution is solidified into a porous thin film by immersion in a bath containing a nonsolvent.

[0003] Asymmetric MF/UF membranes are used in many applications involving chemical, environmental, biological and medical engineering due to their unique separation capability, easy to scale-up possibilities, low energy consumption and recoverability.

(Ghosh, R. J. Chromatogr. A 2002, 952, 13-27. Feins, M.; Sirkar, K. K. Biotechnol. Bioeng. 2004, 86, 603-611.) However, separation properties of UF membranes are limited and specific to only certain applications. Therefore the development of functionalized UF membranes with electrochemically switchable separation properties could be of great interest. (Weidlich, C; Mangold, K. M. Electrochim. Acta 2011, 56, 3481-3484.) In the case of electrically conducting membranes, an externally applied electrical potential could contribute to the charge density and total electrical potential of the membranes. Therefore, the electrical potential of a conducting membrane could be tuned simply by adjusting the potential applied by an external power source, and thereby, charged solute transport (i.e., selectivity) in theory could be controlled. However, such successful electroactive membranes have so far only been demonstrated with relatively expensive, heavy and rigid metal or ceramic semiconductor membranes. (Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. Vlassiouk, I.; Apel, P. Y.; Dmitriev, S. N.; Healy, K.; Siwy, Z. S. Proc. Natl. Acad. Sci. USA 2009, 106, 21039-21044. Luechinger, N. A.; Walt, S. G.; Stark W. J. Chem. Mater. 2010, 22, 4980^1986.) Therefore, simple and scalable routes to light, low-cost, and flexible MF/UF membranes with tunable electroactivites are clearly desirable.

[0004] Conventional MF/UF membranes and methods for preparing same, however, typically lack control over permeability, selectivity, fouling, and cleaning. Therefore, there remains a need for ultrafiltration membranes that overcome these deficiencies and that effectively provide external control over these attributes. Consequently, electro-active switchable ultrafiltration membranes are of great interest due to the possibility of external control over permeability, selectivity, fouling, and cleaning.

SUMMARY

[0005] In accordance with the purpose(s) of the invention, as embodied and broadly described herein, the invention, in one aspect, relates to semi-permeable filtration membranes comprising carbon nanotube-templated conjugated polymer nanofiber composites for use in, for example, cross-flow electro-filtration applications.

[0006] Disclosed are methods for making a semi-permeable filtration membrane, the method comprising the steps of: forming a carbon nanotube-templated conjugated polymer nanofiber composite; optionally, combining the composite with a matrix polymer; solution casting the composite and the optional matrix polymer, thereby providing a membrane; and flash welding at least a portion of a surface of the membrane.

[0007] Also disclosed are methods for making a semi-permeable filtration membrane, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; solution casting the composite onto a nonwoven support fabric, thereby providing a membrane; and flash welding at least a portion of a surface of the membrane.

[0008] Also disclosed are semi-permeable filtration membranes prepared by the disclosed methods.

[0009] Also disclosed are semi-permeable filtration membranes comprising a solid mixture of carbon nanotubes and conjugated polymer formed on a support fabric. [0010] Also disclosed are semi-permeable filtration membranes comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite solution cast onto a nonwoven support fabric.

[0011] Also disclosed are methods for compatibilizing carbon nanotubes, the method comprising the steps of: providing carbon nanotubes; polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube-templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer.

[0012] Also disclosed are methods for compatibilizing carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer.

[0013] Also disclosed are methods for modifying the conductivity of carbon nanotubes, the method comprising the steps of: providing carbon nanotubes; polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube- templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a pH-dependent conductivity.

[0014] Also disclosed are methods for modifying the conductivity of carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a pH-dependent conductivity.

[0015] Also disclosed are methods for tuning selectivity in a semi-permeable filtration membrane, the method comprising the steps of: providing a semi-permeable filtration membrane comprising a conjugated polymer; flash welding at least a portion of a surface of the membrane, thereby changing the membrane selectivity compared to the membrane before performing the flash welding step.

[0016] Also disclosed are methods for tuning selectivity in a semi-permeable filtration membrane, the method comprising the step of flash welding at least a portion of a surface of a membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite, thereby changing the membrane selectivity compared to the membrane before performing the flash welding step.

[0017] Also disclosed are methods for tuning flux in a semi-permeable filtration membrane, the method comprising the steps of: providing a semi-permeable filtration membrane comprising a conjugated polymer; flash welding at least a portion of a surface of the membrane, thereby changing the membrane flux compared to the membrane before performing the flash welding step.

[0018] Also disclosed are methods for tuning flux in a semi-permeable filtration membrane, the method comprising the step of flash welding at least a portion of a surface of a membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite, thereby changing the membrane flux compared to the membrane before performing the flash welding step.

[0019] Also disclosed are methods for purifying water, the methods comprising:

providing a disclosed semi-permeable filtration membrane, or a membrane prepared by a disclosed method, the membrane having a first face and a second face; contacting the first face of the membrane with a first solution of a first volume having a first salt concentration at a first pressure; and contacting the second face of the membrane with a second solution of a second volume having a second salt concentration at a second pressure, wherein the first solution is in fluid communication with the second solution through the membrane, wherein the first salt concentration is higher than the second salt concentration, thereby creating an osmotic pressure across the membrane, and wherein the first pressure is sufficiently higher than the second pressure to overcome the osmotic pressure, thereby increasing the second volume and decreasing the first volume.

[0020] While aspects of the present invention can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present invention can be described and claimed in any statutory class. Unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.

BRIEF DESCRIPTION OF THE FIGURES

[0021] The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate several aspects and together with the description serve to explain the principles of the invention.

[0022] Figure 1 shows Schematic representation of synthetic routes to (a) SWCNT- templated PANi nanofibers and (b) flash welded ultrafiltration membranes, a clear contrast between the welded (uncovered) and unwelded (covered) parts can be seen in the top-right picture.

[0023] Figure 2 shows SEM images of (a) purified SWCNTs and (a, inset) raw SWCNTs; (b) SWCNT-templated PANi nanofibers and (c,d) TEM images of SWCNT-templated PANi nanofibers at (c) low and (d) high magnifications.

[0024] Figure 3 shows ATR/FT-IR spectra of (a) raw and (b) oxidized SWCNTs.

[0025] Figure 4 shows SEM images of SWCNT/PANi composite membrane surfaces of (a) original and (b) flash welded at 100% intensity; and (c) cross-sectional and (d) sub- structural SEM images of the flash-welded membrane.

[0026] Figure 5 shows SEM images of SWCNT/PANi composite membrane surfaces flash welded at the following percent intensities: (a) 6.25, (b) 12.5, (c) 25, (d) 50, (e) 75 and (f) 100%. Scale bar: 1000 nm.

[0027] Figure 6 shows SEM images of SWCNT/PANi composite membrane surfaces flash welded the following number of times: (a) 0, (b) 1, (c) 2, (d) 3, (e) 4 and (f) 5 at full power intensity.

[0028] Figure 7 shows (a,b) Effect on the number of flash welds on the ATR/FT-IR spectra of SWCNT/PANi composite membrane flash welded the following number of times: 0, 1, 2, 3, 4 and 5 at a full power intensity in (a) the high and (b) the low frequency ranges; (c) a schematic illustration of the proposed cross-linking process for the PANi EB nanofibers in the composite.

[0029] Figure 8 shows Effect of the number of flash welds and the flash intensity on the sheet resistance of the SWCNT/PANi EB composite membranes.

[0030] Figure 9 shows Effect of the flash welding intensity on the (a) S1O₂ nanoparticle rejection, (b) BSA nanoparticle rejection and (c) permeability (i.e., pure water flux) of the SWCNT/PANi composite membranes.

[0031] Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or can be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

DESCRIPTION

[0032] The present invention can be understood more readily by reference to the following detailed description of the invention and the Examples included therein.

[0033] Before the present compounds, compositions, articles, systems, devices, and/or methods are disclosed and described, it is to be understood that they are not limited to specific synthetic methods unless otherwise specified, or to particular reagents unless otherwise specified, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, example methods and materials are now described.

[0034] All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.

A. DEFINITIONS

[0035] As used herein, nomenclature for compounds, including organic compounds, can be given using common names, IUPAC, IUBMB, or CAS recommendations for

nomenclature. When one or more stereochemical features are present, Cahn-Ingold-Prelog rules for stereochemistry can be employed to designate stereochemical priority, EIZ specification, and the like. One of skill in the art can readily ascertain the structure of a compound if given a name, either by systemic reduction of the compound structure using naming conventions, or by commercially available software, such as CHEMDRAW™ (Cambridgesoft Corporation, U.S.A.).

[0036] As used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a functional group," "an alkyl," or "a residue" includes mixtures of two or more such functional groups, alkyls, or residues, and the like.

[0037] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms a further aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as "about" that particular value in addition to the value itself. For example, if the value "10" is disclosed, then "about 10" is also disclosed. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 1 1, 12, 13, and 14 are also disclosed.

[0038] References in the specification and concluding claims to parts by weight of a particular element or component in a composition denotes the weight relationship between the element or component and any other elements or components in the composition or article for which a part by weight is expressed. Thus, in a compound containing 2 parts by weight of component X and 5 parts by weight component Y, X and Y are present at a weight ratio of 2:5, and are present in such ratio regardless of whether additional components are contained in the compound.

[0039] A weight percent (wt. %) of a component, unless specifically stated to the contrary, is based on the total weight of the formulation or composition in which the component is included.

[0040] As used herein, the terms "optional" or "optionally" means that the subsequently described event or circumstance can or can not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0041] A residue of a chemical species, as used in the specification and concluding claims, refers to the moiety that is the resulting product of the chemical species in a particular reaction scheme or subsequent formulation or chemical product, regardless of whether the moiety is actually obtained from the chemical species. Thus, an ethylene glycol residue in a polyester refers to one or more -OCH₂CH₂0- units in the polyester, regardless of whether ethylene glycol was used to prepare the polyester. Similarly, a sebacic acid residue in a polyester refers to one or more -CO(CH₂)₈CO- moieties in the polyester, regardless of whether the residue is obtained by reacting sebacic acid or an ester thereof to obtain the polyester.

[0042] As used herein, the term "polymer" refers to a relatively high molecular weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, the monomer (e.g., polyethylene, rubber, cellulose). Synthetic polymers are typically formed by addition or condensation polymerization of monomers.

[0043] As used herein, the term "copolymer" refers to a polymer formed from two or more different repeating units (monomer residues). By way of example and without limitation, a copolymer can be an alternating copolymer, a random copolymer, a block copolymer, or a graft copolymer. It is also contemplated that, in certain aspects, various block segments of a block copolymer can themselves comprise copolymers.

[0044] As used herein, the term "oligomer" refers to a relatively low molecular weight polymer in which the number of repeating units is between two and ten, for example, from two to eight, from two to six, or form two to four. In one aspect, a collection of oligomers can have an average number of repeating units of from about two to about ten, for example, from about two to about eight, from about two to about six, or form about two to about four.

[0045] As used herein, the term "molecular weight" (MW) refers to the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon- 12).

[0046] As used herein, the term "number average molecular weight" (M_n) refers to the common, mean, average of the molecular weights of the individual polymers. M_n can be determined by measuring the molecular weight of n polymer molecules, summing the weights, and dividing by n. M_n is calculated by:

wherein Ni is the number of molecules of molecular weight Mi. The number average molecular weight of a polymer can be determined by gel permeation chromatography, viscometry (Mark-Houwink equation), light scattering, analytical ultracentrifugation, vapor pressure osmometry, end-group titration, and colligative properties.

[0047] As used herein, the term "weight average molecular weight" (M_w) refers to an alternative measure of the molecular weight of a polymer. M_w is calculated by:

wherein Ni is the number of molecules of molecular weight Mi. Intuitively, if the weight average molecular weight is w, and a random monomer is selected, then the polymer it belongs to will have a weight of w, on average. The weight average molecular weight can be determined by light scattering, small angle neutron scattering (SANS), X-ray scattering, and sedimentation velocity.

[0048] As used herein, the terms "polydispersity" and "polydispersity index" refer to the ratio of the weight average to the number average (M_w/M_n). [0049] As used herein, the terms "flash welding" and "flash weld" refer to applying a pulse of light to an absorbing material. Flash welding can provide enhanced photothermal phenomena when performed on polymeric nano fibers. In certain aspects, the material rapidly converts the light to heat and then undergoes a transformation, such as melting. It is understood that, in certain aspects, chemical reactions can take place in the material as a consequence of flash welding (see, e.g., Figure 7). Techniques for performing flash welding are described in U.S. Patent No. 7,850,798 ("Flash welding of conducting polymers nanofibers"), issued December 14, 2010, to J. Huang and R. B. Kaner.

[0050] Certain materials, compounds, compositions, and components disclosed herein can be obtained commercially or readily synthesized using techniques generally known to those of skill in the art. For example, the starting materials and reagents used in preparing the disclosed compounds and compositions are either available from commercial suppliers such as Aldrich Chemical Co., (Milwaukee, Wis.), Acros Organics (Morris Plains, N.J.), Fisher Scientific (Pittsburgh, Pa.), or Sigma (St. Louis, Mo.) or are prepared by methods known to those skilled in the art following procedures set forth in references such as Fieser and Fieser's Reagents for Organic Synthesis, Volumes 1-17 (John Wiley and Sons, 1991); Rodd's Chemistry of Carbon Compounds, Volumes 1-5 and Supplemental volumes (Elsevier Science Publishers, 1989); Organic Reactions, Volumes 1-40 (John Wiley and Sons, 1991); March's Advanced Organic Chemistry, (John Wiley and Sons, 4th Edition); and Larock's

Comprehensive Organic Transformations (VCH Publishers Inc., 1989).

[0051] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order.

Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; and the number or type of embodiments described in the specification.

[0052] Disclosed are the components to be used to prepare the compositions of the invention as well as the compositions themselves to be used within the methods disclosed herein. These and other materials are disclosed herein, and it is understood that when combinations, subsets, interactions, groups, etc. of these materials are disclosed that while specific reference of each various individual and collective combinations and permutation of these compounds can not be explicitly disclosed, each is specifically contemplated and described herein. For example, if a particular compound is disclosed and discussed and a number of modifications that can be made to a number of molecules including the compounds are discussed, specifically contemplated is each and every combination and permutation of the compound and the modifications that are possible unless specifically indicated to the contrary. Thus, if a class of molecules A, B, and C are disclosed as well as a class of molecules D, E, and F and an example of a combination molecule, A-D is disclosed, then even if each is not individually recited each is individually and collectively

contemplated meaning combinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considered disclosed. Likewise, any subset or combination of these is also disclosed. Thus, for example, the sub-group of A-E, B-F, and C-E would be considered disclosed. This concept applies to all aspects of this application including, but not limited to, steps in methods of making and using the compositions of the invention. Thus, if there are a variety of additional steps that can be performed it is understood that each of these additional steps can be performed with any specific embodiment or combination of embodiments of the methods of the invention.

[0053] It is understood that the compositions disclosed herein have certain functions. Disclosed herein are certain structural requirements for performing the disclosed functions, and it is understood that there are a variety of structures that can perform the same function that are related to the disclosed structures, and that these structures will typically achieve the same result.

B. MEMBRANES FOR CROSS-FLOW ELECTRO-FILTRATION APPLICATIONS

[0054] Hybrid single-walled carbon nanotube (SWCNT)-polyaniline (PANi) nanofibers have been synthesized by in situ polymerization of aniline in the presence of oxidized SWCNTs. The composite nanofibers exhibit unique morphology of core-shell

(SWCNT-PANi) structures with average total diameters of 60 nm with 10 to 30 nm thick PANi coating. The composite nanofibers were dispersed in polar aprotic solvents and cast into asymmetric membranes via a nonsolvent induced phase separation. The hybrid

SWCNT/PANi membranes were electrically conductive at neutral pH and exhibited ultrafiltration-like permeability and selectivity when filtering aqueous suspensions of bovine serum albumin and 48 nm diameter silica particles. A novel flash welding technique is utilized to tune the morphology, porosity, conductivity, permeability and nanoparticle rejection of the SWCNT/PANi composite ultrafiltration membranes. Upon flash welding, both conductivity and pure water permeability of the membranes improves by nearly a factor of 10, while maintaining silica nanoparticle rejection levels above 90%. Flash welding of SWCNT/PANi composite membranes show promise for formation of electrochemically tunable membranes.

[0055] Single-walled carbon nanotubes (SWCNTs) possess excellent mechanical and electronic properties, e.g., they can carry an electrical current density up to 4 x 10⁹ A cm^"2, which is 3 orders of magnitude higher than typical metals such as aluminum or copper. (Hong, S. H.; Myung, S. Nat. Nanotech.2007 , 2, 207-208.) The Young's modulus of a SWCNT ranges from 0.32 to 1.47 TPa with strengths between 10 and 52 GPa and a toughness of 770 J g^"1. (Yu, M. F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. Rev.

e//.2000, 84, 5552-5555.) SWCNTs usually exhibit diameters of 1-2 nm and lengths of a few micrometers or more. Carbon nanotubes are thus good candidates to serve as both mechanical and electrical reinforcements for light-weight composites with a low percolation threshold (G. Nechifor, S. I. Voicu, A. C. Nechifor and S. Garea, Desalination 2009, 241, 342.), especially for hybrid separation membranes. (L. J. Sweetman, L. Nghiem, I. Chironi,

G. Triani, M. in het Panhuis and S. F. Ralph, J. Mater. Chem. 2012, 22, 13800. T.-H. Weng,

H. -H. Tseng and M.-Y. Wey, Int. J. Hydrogen Energ. 2009, 34, 8707.)

[0056] Polyaniline (PANi), a prototypical conjugated polymer, has been extensively studied for potential applications in electronic devices due to its facile synthesis,

environmental stability, unique electronic properties and simple acid/base doping/dedoping chemistry. (Li, D.; Huang, J. X.; Kaner, R. B. Acc. Chem. Res.2009, 42, 135-145. Tran, H. D.; Li, D.; Kaner, R. B. Adv. Mater.2009, 21, 1487-1499.) PANi nanofibers have demonstrated enhanced performance in applications such as chemical sensors (Huang, J. X.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc.2003, 125, 314-315.) supercapacitors (Wu, Q.; Xu, Y. X.; Yao. Z. Y.; Liu, A. R.; Shi, G. Q. ACSNanolQlQ, 4, 1963-1970.), catalyst supports (Han, J.; Li, L. Y.; Guo, R. Macromolecules2010, 43, 10636-10644.), and membrane devices (Fan, Z. F.; Wang, Z.; Sun, N.; Wang, J. X.; Wang, S. C. J. Membr.

& .2008, 320, 363-367.) due to their high surface area, porous nature and hydrophilicity. In particular, the PANi nanofibers possess a unique property amongst the conducting polymers in that films can be patterned by both flash and laser welding. (Huang, J. X.; Kaner, R. B. Nat. Mater.2004, 3, 783-786. Strong, V.; Wang, Y.; Patatanyan, A.; Whitten, P. G.; Spinks, G. M.; Wallace, G. G.; Kaner, R. B. Nano Lett. 2011, 11, 3128-3135.) During welding, the nanofiber networks are exposed to short bursts of high intensity light, changing their morphology, conductivity, doping and spectroscopic properties. The flash welding technique exploits the photothermal response of PANi nano fibers thus opening new avenues for processing and patterning polymer-based materials that could lead to electronic devices and asymmetric membranes. (Huang, J. X.; Kaner, R. B. Nat. Mater.2004, 3, 783-786. Strong, V.; Wang, Y.; Patatanyan, A.; Whitten, P. G.; Spinks, G. M.; Wallace, G. G.; Kaner, R. B. Nano Lett.2011, 11, 3128-3135. Li, D.; Xia, Y. N. Nat. Mater.2004, 3, 753-754.)

[0057] By integrating the excellent properties of PANi and SWCNTs, it is possible to combine the advantages of both for the development of new nanotechnologies, electronics and separation devices. For PANi, polymer molecules orient along the carbon nanotubes producing high surface area materials. For SWCNTs, the addition of polymers can be helpful for improving the processing of composites. Despite a variety of in situ polymerization techniques that have been tried for creating such composites (Wei, Z. X.; Wan, M. X.; Lin, T.; Dai, L. M. Adv. Mater.2003, 15, 136-139. W. R. Small, F. Masdarolomoor, G. G. Wallace and M. int het Panhuis. J. Mater. Chem. 2007, 17, 4359. Ginic-Markovic, M.; Matisons, J. G.; Cervini, R.; Simon, G. P.; Fredericks, P. M. Chem. Mater.2006, 18, 6258-6265.

Salvatierra, R. V.; Oliveira, M. M.; Zarbin, A. J. G. Chem. Mater.2010, 22, 5222-5234. Yao, Q.; Chen, L. D.; Zhang, W. Q.; Liufu, S. C; Chen, X. H. ACSNano20lO, 4, 2445-2451.), the synthesis of SWCNT/PANi composite materials with bulk one-dimensional (ID)

nanostructures, good dispersibility, high conductivity and low SWCNT loading have so far met with only limited success. Recently, it was reported an initiator-assisted chemical processing technique that produces nanofibers of PANi coated on SWCNTs. (Liao, Y. Z.; Zhang, C; Zhang, Y.; Strong, V.; Tang, J. S.; Li, X. G.; Hoek, E. M. V.; Wang, K. L.; Kaner, R. B. NanoLett.2011, 11, 954-959.) The resulting materials exhibit exceptional conductivity and mobility and are remarkably easy to prepare in surfactant- free water dispersions. (Liao, Y. Z.; Zhang, C; Wang, X.; Li, X. G.; Ippolito, S. J.; Kalantar-zadeh, K.; Kaner, R. B. J. Phys. Chem. C 2011, 115, 16187-16192.)

[0058] The present application discloses a novel technique to synthesize ID

nanocomposites comprised of SWCNT-templated PANi nanofibers in the absence of an initiator. In order to obtain more reaction sites and higher miscibility between the two components, the SWCNTs are oxidized by strong acids (H₂SO₄ and HNO₃) to introduce active carboxyl groups, and then applied as templates for coating uniform PANi layers. The SWCNT-templated PANi nanofibers are further solvent-processed thereby producing neutral- conductive and asymmetric UF membranes using a simple nonsolvent induced phase separation (NIPS) technique. Additionally, flash welding is utilized to tune the morphology, conductivity, permeability and nanoparticle rejection of the SWCNT/PANi composite UF membranes. The effects of varying the percent power and number of flash welds on the conductivity as well as separation properties including pure water flux, silica (S1O₂) and bovine serum albumin (BSA) nanoparticle rejection of the electrically conducting

SWCNTs/PANi UF membranes are studied. The combination of high electrical conductivity, porosity, hydrophilicity, permeability and selectivity of the SWCNT/PANi composite membranes show promise for cross-flow electro-filtration applications.

C. SEMI-PERMEABLE FILTRATION MEMBRANES

[0059] In one aspect, the invention relates to a semi-permeable filtration membrane comprising a solid mixture of carbon nanotubes and conjugated polymer formed on a support fabric. In a further aspect, the invention relates to a semi-permeable filtration membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite solution cast onto a nonwoven support fabric.

[0060] In a further aspect, the membrane has been subjected to flash welding. In a further aspect, the membrane is formed by polymerization of aromatic monomers in the presence of carbon nanotubes. In a further aspect, the membrane is produced by solution casting. In a further aspect, solution casting is nonsolvent induced phase separation or evaporation. In a further aspect, the support fabric is nonwoven polyester.

1. CARBON NANOTUBES

[0061] In a further aspect, the carbon nanotubes are single-walled. In a further aspect, the carbon nanotubes have been oxidized prior to incorporation into the membrane.

[0062] In various aspects, the carbon nanotubes can have a diameter distribution peaked at from about 1.0 nm to about 2.0 nm. For example, the diameter can be peaked at about 1.0 nm, about 1.2 nm, about 1.4 nm, about 1.6 nm, about 1.8 nm, or about 2.0 nm. [0063] In various aspects, the carbon nanotubes can have a length-to-diameter ratio of from about 10,000: 1 to about 100,000,000: 1. For example, the ratio can be from about 10,000: 1 to about 100,000: 1, about 100,000: 1 to about 1,000,000: 1, about 1,000,000: 1 to about 10,000,000: 1, or about 10,000,000: 1 to about 100,000,000: 1.

2. CONJUGATED POLYMER

[0064] In one aspect, the conjugated polymer is selected from polyaniline, polythiophene, and polypyrrole. In a further aspect, the conjugated polymer is polyaniline. In a further aspect, the conjugated polymer is produced by chemical oxidative polymerization in the presence of carbon nanotubes. In a further aspect, the conjugated polymer is polyaniline in the dedoped emeraldine oxidation state.

3. NANOTUBE-TEMPLATED CO JUGATED POLYMER NANOFIBER COMPOSITE

[0065] In one aspect, the invention relates to membranes comprising a nanotube- templated conjugated polymer nanofiber composite. In a further aspect, the nanotubes are carbon nanotubes, which can be single-walled. In a further aspect, the conjugated polymer can comprise polyaniline. In a further aspect, the nanotubes can be oxidized prior to formation of the composite. In a further aspect, the composite can comprise oxidized carbon nanotube-templated polyaniline nanofibers.

[0066] In one aspect, a nanotube-templated conjugated polymer nanofiber composite can be formed by polymerization of an aromatic monomer (e.g., aniline) to form a conjugated polymer (e.g., polyaniline) in the presence of nanotubes (e.g., carbon nanotubes, single- walled carbon nanotubes, or oxidized single-walled carbon nanotubes).

4. MATRIX POLYMER

[0067] In a further aspect, the mixture further comprises a matrix polymer. In a further aspect, the matrix polymer is selected from polyaniline, polythiophene, polypyrrole, polysulfone (PSf) , polyethersulfone (PES), sulfonated PSf, sulfonated PES, polyacrilonitrile, cellulose, nitrocellulose, regenerated cellulose, cellulose acetate, cellulose diacetate, cellulose triacetate, polycarbonate, polyamides, polyimides, PBI, polyetherketone,

polyetheretherketone (PEEK), sulfonated PEEK, polyphenylsulfides, polyvinylidine fluoride, and polyterephthaloyl fluoride. 5. POLYMER FILM

[0068] In a further aspect, the membrane further comprises a polyamide thin film interfacially polymerized on a surface of membrane. That is, in certain aspects, the membranes can be reverse osmosis membranes.

6. FLUX (PERMEABILITY)

[0069] The pure water flux of the membranes can be measured in a laboratory scale cross-flow membrane filtration apparatus. For example, the pure water flux can be measured in a high-pressure chemical resistant stirred cell (Sterlitech HP4750 Stirred Cell). In one aspect, the membranes can have a flux of from about 80 to about 400 GFD (gallons per square foot of membrane per day) per psi (pound per square inch) of applied pressure.

[0070] For example, the flux can be from about 100 to about 400, from about 100 to about 200, from about 200 to about 300, from about 300 to about 400, from about 150 to about 250, from about 250 to about 350, or from about 80 to about 200 gallons per square foot of membrane per day per psi of applied pressure.

7. REJECTION (SELECTIVITY)

[0071] Rejection, or selectivity, of the membranes can depend upon the species being filtered. For example, the rejection of a particular membrane can be higher for S1O₂ particles than for BSA nanoparticles.

[0072] In one aspect, a membrane of the invention can have a S1O₂ particle rejection of from about 80% to greater than about 95%. For example, the S1O₂ particle rejection can be from about 80% to about 85%, from about 85% to about 90%, from about 90% to about 95%, or greater than about 95%.

[0073] In a further aspect, a membrane of the invention can have a BSA nanoparticle rejection of from about 5% to about 35%. For example, the BSA nanoparticle rejection can be from about 5% to about 10%, from about 10% to about 15%, from about 15% to about 20%, from about 20% to about 25%, from about 25% to about 30%, or from about 30% to about 35%. 8. RESISTANCE TO FOULING

[0074] The relative biofouling potentials of the membranes of the invention can be evaluated by direct microscopic observation of microbial deposition and adhesion. (S. Kang, A. Subramani, E.M.V. Hoek, M.R. Matsumoto, and M.A. Deshusses, Direct observation of biofouling in cross-flow microfiltration: mechanisms of deposition and release, Journal of Membrane Science 244 (2004) 151-165.) Viability of bacteria adhered to membranes can be verified with a commercial viability staining kit (e.g., LIVE/DEAD^® BacLight™ Bacterial Viability Kit, Molecular Probes, Inc., Eugene Oregon) for 2-4 minutes, followed by observation using a fluorescence microscope (e.g., BX51, Olympus America, Inc., Melville, N.Y.). Living cells can be observed as green spots and dead (inactivated) cells are seen as red spots. (B.K. Li and B.E. Logan, The impact of ultraviolet light on bacterial adhesion to glass and metal oxide-coated surface, Colloids and Surfaces B-Biointerfaces 41 (2005) 153- 161.)

9. SHAPE

[0075] A variety of membrane shapes are useful and can be provided using the present invention. These include spiral wound, hollow fiber, tubular, or flat sheet type membranes.

10. PH-DEPENDENT CONDUCTIVITY

[0076] In one aspect, the membranes have a pH-dependent conductivity. That is, the nanofibers composites within the membranes can have a conductivity that varies as a function of the environmental pH. For example, the conductivity can increase as pH increases. As another example, the conductivity can decrease as pH increases.

[0077] It is understood that the disclosed membranes can be provided by using the disclosed methods of making. It is also understood that the disclosed membranes can be used in the disclosed methods of using.

D. METHODS FOR MAKING A SEMI-PERMEABLE FILTRATION MEMBRANES

[0078] In one aspect, the invention relates to a method for making a semi-permeable filtration membrane, the method comprising the steps of: forming a carbon nanotube- templated conjugated polymer nanofiber composite; optionally, combining the composite with a matrix polymer; solution casting the composite and the optional matrix polymer, thereby providing a membrane; and flash welding at least a portion of a surface of the membrane.

[0079] In one aspect, the invention relates to a method for making a semi-permeable filtration membrane, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; solution casting the composite onto a nonwoven support fabric, thereby providing a membrane; and flash welding at least a portion of a surface of the membrane.

[0080] In a further aspect, the method further comprises the step of oxidizing the carbon nanotubes prior to the polymerization step. In a further aspect, the method further comprises the step of isolating the composite before solution casting. In a further aspect, the method further comprises the step of interfacially polymerizing a polyamine and a polyfunctional acyl halide on a surface of the membrane to form a reverse osmosis membrane.

[0081] In a further aspect, forming is polymerization of aromatic monomers in the presence of carbon nanotubes. In a further aspect, the carbon nanotubes are single-walled. In a further aspect, the aromatic monomers are selected from aniline, thiophene, and pyrrole. In a further aspect, the aromatic monomers comprise aniline. In a further aspect, polymerization is chemical oxidative polymerization. In a further aspect, the polyaniline is in the dedoped emeraldine oxidation state.

[0082] In a further aspect, the matrix polymer is present and is selected from polyaniline, polythiophene, polypyrrole, polysulfone, polyethersulfone, sulfonated PSf, sulfonated PES, polyacrilonitrile, cellulose, nitrocellulose, regenerated cellulose, cellulose acetate, cellulose diacetate, cellulose triacetate, polycarbonate, polyamides, polyimides, PBI, polyetherketone, polyetheretherketone, sulfonated PEEK, polyphenylsulfides, polyvinylidine fluoride, and polyterephthaloyl fluoride.

[0083] In a further aspect, solution casting is nonsolvent induced phase separation. In a further aspect, solution casting is evaporation. In a further aspect, the membrane is cast onto a nonwoven support fabric.

[0084] It is understood that the disclosed methods can be used to provide the disclosed membranes. E. METHODS FOR COMPATIBILIZING CARBON NANOTUBES

[0085] Polymerization of an aromatic monomer (e.g., aniline) to form a conjugated polymer (e.g., polyaniline) in the presence of nanotubes (e.g., carbon nanotubes, single- walled carbon nanotubes, or oxidized single-walled carbon nanotubes) can be employed to make the nanotubes more compatible with a matrix polymer. The matrix polymer can be the same or different from the conjugated polymer.

[0086] Encapsulation of nanotubes with polyaniline renders them biocompatible and nontoxic in addition to making them dispersible in water and polar organic solvents such as are commonly used in preparation of membrane materials.

[0087] Thus, in one aspect, the invention relates to a method for compatibilizing carbon nanotubes, the method comprising the steps of: providing carbon nanotubes; polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube-templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer. In a further aspect, the invention relates to a method for compatibilizing carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube- templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer.

[0088] In a further aspect, the polymer formed in the polymerization step has the same chemical backbone as the matrix polymer. In a further aspect, the aromatic monomers are selected from aniline, thiophene, and pyrrole. In a further aspect, the aromatic monomers comprise aniline. In a further aspect, the conjugated polymer is polyaniline in the dedoped emeraldine oxidation state.

[0089] In a further aspect, the matrix polymer is employed and is selected from polyaniline, polythiophene, polypyrrole, polysulfone, polyethersulfone, sulfonated PSf, sulfonated PES, polyacrilonitrile, cellulose, nitrocellulose, regenerated cellulose, cellulose acetate, cellulose diacetate, cellulose triacetate, polycarbonate, polyamides, polyimides, PBI, polyetherketone, polyetheretherketone, sulfonated PEEK, polyphenylsulfides, polyvinylidine fluoride, and polyterephthaloyl fluoride. [0090] It is understood that the disclosed compatibilization methods can be used in connection with the disclosed methods of making. It is also understood that the disclosed compatibilization methods can also be used in connection with preparing devices and articles comprising nanotubes, other than membranes.

F. METHOD FOR MODIFYING THE CONDUCTIVITY OF CARBON NANOTUBES

[0091] Polymerization of an aromatic monomer (e.g., aniline) to form a conjugated polymer (e.g., polyaniline) in the presence of nanotubes (e.g., carbon nanotubes, single- walled carbon nanotubes, or oxidized single-walled carbon nanotubes) can be employed to modify the conductivity of the nanotubes. That is, the resultant nanotube-templated conjugated polymer nanofiber composite can have a conductivity greater than, or a conductivity less than, the conductivity of the nanotubes alone. Moreover, the resultant nanotube-templated conjugated polymer nanofiber composite can have a pH-dependent conductivity.

[0092] Thus, in one aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of: providing carbon nanotubes; polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube-templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a pH-dependent conductivity. Thus, in one aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of:

providing carbon nanotubes; polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube-templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a conductivity greater than the conductivity of the nanotubes alone. Thus, in one aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of: providing carbon nanotubes;

polymerizing aromatic monomers in the presence of the carbon nanotubes, thereby forming a carbon nanotube-templated conjugated polymer nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a conductivity less than the conductivity of the nanotubes alone. [0093] In a further aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube- templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a pH-dependent conductivity. In a further aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a conductivity greater than the conductivity of the nanotubes alone. In a further aspect, the invention relates to a method for modifying the conductivity of carbon nanotubes, the method comprising the steps of: polymerizing aniline in the presence of oxidized carbon nanotubes, thereby forming an oxidized carbon nanotube-templated polyaniline nanofiber composite; and optionally, combining the composite with a matrix polymer, wherein the product formed has a conductivity less than the conductivity of the nanotubes alone.

[0094] In a further aspect, the polymer formed in the polymerization step has the same chemical backbone as the matrix polymer. In a further aspect, the aromatic monomers are selected from aniline, thiophene, and pyrrole. In a further aspect, the aromatic monomers comprise aniline. In a further aspect, the conjugated polymer is polyaniline in the dedoped emeraldine oxidation state.

[0095] In a further aspect, the matrix polymer is employed and is selected from polyaniline, polythiophene, polypyrrole, polysulfone, polyethersulfone, sulfonated PSf, sulfonated PES, polyacrilonitrile, cellulose, nitrocellulose, regenerated cellulose, cellulose acetate, cellulose diacetate, cellulose triacetate, polycarbonate, polyamides, polyimides, PBI, polyetherketone, polyetheretherketone, sulfonated PEEK, polyphenylsulfides, polyvinylidine fluoride, and polyterephthaloyl fluoride.

[0096] It is understood that the disclosed modification methods can be used in connection with the disclosed methods of making. It is also understood that the disclosed modification methods can also be used in connection with preparing devices and articles comprising nanotubes, other than membranes. G. METHODS FOR TUNING SELECTIVITY IN SEMI-PERMEABLE FILTRATION MEMBRANES

[0097] Flash welding of membranes comprising conjugated polymer, or a nanotube- templated conjugated polymer nanofiber composite, can tune the selectivity (rejection) of the membrane. For example, the selectivity can be decreased by increasing the amount and/or increasing the intensity of the flash welding.

[0098] Thus, in one aspect, the invention relates to a method for tuning selectivity in a semi-permeable filtration membrane, the method comprising the steps of: providing a semipermeable filtration membrane comprising a conjugated polymer; flash welding at least a portion of a surface of the membrane, thereby changing the membrane selectivity compared to the membrane before performing the flash welding step. In a further aspect, the membrane further comprises carbon nanotubes. In a further aspect, selectivity decreases.

[0099] In a further aspect, the invention relates to a method for tuning selectivity in a semi-permeable filtration membrane, the method comprising the step of flash welding at least a portion of a surface of a membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite, thereby changing the membrane selectivity compared to the membrane before performing the flash welding step.

[00100] It is understood that the disclosed selectivity tuning methods can be used in connection with the disclosed methods of making. It is also understood that the disclosed selectivity tuning methods can also be used in connection with preparing devices and articles comprising nanotubes, other than membranes.

H. METHODS FOR TUNING FLUX IN SEMI-PERMEABLE FILTRATION MEMBRANES

[00101] Flash welding of membranes comprising conjugated polymer, or a nanotube- templated conjugated polymer nanofiber composite, can tune the flux (permeability) of the membrane. For example, the flux can be increased by increasing the amount and/or increasing the intensity of the flash welding.

[00102] Thus, in one aspect, the invention relates to a method for tuning flux in a semipermeable filtration membrane, the method comprising the steps of: providing a semipermeable filtration membrane comprising a conjugated polymer; flash welding at least a portion of a surface of the membrane, thereby changing the membrane flux compared to the membrane before performing the flash welding step. In a further aspect, the membrane further comprises carbon nanotubes. In a further aspect, flux increases.

[00103] In a further aspect, the invention relates to a method for tuning flux in a semipermeable filtration membrane, the method comprising the step of flash welding at least a portion of a surface of a membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite, thereby changing the membrane flux compared to the membrane before performing the flash welding step.

[00104] It is understood that the disclosed flux tuning methods can be used in connection with the disclosed methods of making. It is also understood that the disclosed flux tuning methods can also be used in connection with preparing devices and articles comprising nanotubes, other than membranes.

I. METHODS FOR PURIFYING WATER WITH SEMI-PERMEABLE MEMBRANES

[00105] The invention can be used as a filtration membrane for performing water purification, bioseparations, protein purification, oil-water separations, etc.

[00106] Thus, in one aspect, the invention relates to a method for purifying water, the method comprising: providing a disclosed semi-permeable filtration membrane, the membrane having a first face and a second face; contacting the first face of the membrane with a first solution of a first volume having a first salt concentration at a first pressure; and contacting the second face of the membrane with a second solution of a second volume having a second salt concentration at a second pressure, wherein the first solution is in fluid communication with the second solution through the membrane, wherein the first salt concentration is higher than the second salt concentration, thereby creating an osmotic pressure across the membrane, and wherein the first pressure is sufficiently higher than the second pressure to overcome the osmotic pressure, thereby increasing the second volume and decreasing the first volume.

[00107] It is understood that the disclosed purification methods can be used in connection with the disclosed membranes. It is also understood that the disclosed purification methods can be used in connection with the products of the disclosed methods. J. EXPERIMENTAL

[00108] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C or is at ambient temperature, and pressure is at or near atmospheric.

1. MATERIALS

[00109] Raw single-walled carbon nanotubes (SWCNTs, 60-70%) with a narrow diameter distribution peaked at 1.4 nm were purchased from Carbon Solution, Inc. (USA). They were synthesized using a Ni/Y catalyst by chemical vapor deposition. Silica nanoparticles (S1O₂, average diameter of 48 nm) were supplied from Nissan Chemical Corp. (USA). All other chemical reagents including aniline (99.5%), ammonium peroxydisulfate (APS, 98%), bovine serum albumin (BSA, 96%), nitric acid (HN0₃, 68-70%), sulfuric acid (H₂S0₄, 95-98%), methanol (MeOH, 99.6%), N-methylpyrrolidinone (ΝΜΡ, 99%), and sodium hydroxide (NaOH, 98%) were purchased from Sigma-Aldrich and used as received.

2. OXIDATION OF SWCNTs

[00110] In order to improve the purity and dispersibility of SWCNTs, raw SWCNTs were oxidized using a strong acid mixture of HNO₃/H₂SC (1/3, v/v) (Figure la). Typically, 1.5 g of raw SWCNTs were added into the 100 mL mixture of concentrated HNO₃/H₂SC (1/3, v/v) at 60 °C for 6 h, then cooled to room temperature and poured into 1 L of cold de-ionized (DI) water. The product was repeatedly centrifuged and washed with DI water and MeOH until the centrifuged solution reached a neutral pH. Finally, 50 mL of carbon nanotubes as DI water suspension containing ~1.0 g of oxidized SWCNTs was obtained. The suspension was horn sonicated for 30 min before using it as a template for the synthesis of PANi nanofibers. 3. SWCNT-TEMPLATED SYNTHESIS OF PANI NANOFIBERS.

[00111] The SWCNT-templated PANi nanofibers were synthesized by in situ chemical oxidative polymerization of aniline in the oxidized SWCNT suspension (Figure la).

Typically, a monomer solution composed of aniline (20 mL, 0.2194 mol) dissolved in aqueous H2SO4 (0.5 M, 1.0 L) was mixed at room temperature with the above oxidized SWCNT suspension and then magnetically stirred for 4 h. An oxidant solution composed of APS (12 g, 0.0526 mol) dissolved in aqueous H₂S0₄ (0.5 M, 1.0 L) was then poured into the above mixture at room temperature. The reaction was rapidly stirred for 1 h to evenly distribute the oxidant, monomer and SWCNTs and then left standing for 23 h. The product was recovered by microfiltration (0.45 mm Durapore^® membrane, Millipore) and washed with NaOH (1.0 M, 1.0 L) followed by DI water (1.0 L) and MeOH (1.0 L) to produce the dedoped emeraldine oxidation state of polyaniline. A very small amount of the product as DI water dispersion was used for characterization by microscopy. A dried powder (2.51 g) of SWCNT-templated PANi emeraldine base (EB) nanofibers was obtained by drying the samples in vacuum at 50 °C for two days. For comparison, 1.52 g of pure PANi EB was synthesized in the absence of oxidized SWCNTs by following the same technique. Therefore, the oxidized SWCNT content in the final dedoped composite nanofibers is estimated to be approximately 40%, with the assumption that the presence of SWCNTs has a negligible effect on the aniline polymerization yield.

4. PREPARATION AND FLASH WELDING OF SWCNT/PANI COMPOSITE

MEMBRANES.

[00112] The SWCNT/PANi composite membranes were created by a nonsolvent induced phase separation (NIPS) technique using DI water as the coagulation bath (Figure lb). In this process, 0.55 g of SWCNT/PANi EB composite was slowly added into 9.9 g of NMP and magnetically stirred for one day; then another 0.55 g of composite was slowly added and the mixture was stirred for 24 h, forming a homogeneous casting solution at a concentration of 10%. The solution was cast on a commercial nonwoven polyester support fabric (NanoH^O Inc., Los Angeles, California) and then immersed in 18 ΜΩ laboratory DI water. Porous membranes of -100 μιη in thickness were obtained after the solvent/nonsolvent (NMP/water) exchange and induced precipitation of SWCNTs and PANi. To investigate the effect of flash welding on the conductivity, permeability and nanoparticle rejection, dried SWCNT/PANi composite membranes were flash welded by holding the devices 5 cm above the surface of the membranes and applying a full power (640Ws) flash. The power of the flash welding setup (Alienbees B1600 640 Ws purchased from Digital Photography Solutions, USA) can be adjusted from 0 to 100% of full power.

5. CHEMICAL COMPOSITION, MORPHOLOGY CHARACTERIZATION AND PROPERTY MEASUREMENTS.

[00113] The chemical bonding in the raw SWCNTs, oxidized SWCNTs and

SWCNT/PANi EB composite membranes were characterized by using Attenuated Total Reflection/Fourier transform infrared (ATR/FT-IR, JASCO 620) spectroscopy. The morphologies of the samples were imaged using field emission scanning electron microscopy (SEM, JEOL JSM-6700) and transmission electron microscopy (TEM, PHILIPS CM120). The conductivities of the SWCNT/PANi EB composite membranes were obtained by using a two-probe technique as previously reported. (Liao, Y. Z.; Li, X. G.; Kaner, R. B. ACS

Na«o2010, 4, 5193-5202.) The resistances in ohms per square cm (Ω/cm²) of the asymmetric membranes were measured using a two-point probe setup by painting two silver lines of the same length and sequence distance onto the surface. Initial permeability (i.e., pure water flux) and selectivity (i.e., nanoparticle rejection) measurements were conducted in a dead-end ultrafiltration stirred-cell apparatus under a trans-membrane pressure of ~10 psi at 25 °C Silica (S1O₂) and bovine serum albumin (BSA) nanoparticles with average diameters of 48 and 6 nm, respectively, were used to evaluate the selectivity of the membranes. The concentrations of S1O2 and BSA were measured using a Hach 2100Ν Turbidimeter and a HP 8453 UV-vis spectrophotometer, respectively. Particle rejections (r) were calculated from:

Λ ™~ 1 where c_p and c are nanoparticle concentrations in the permeate and feed streams, respectively. (Guillen, G. R.; Farrell, T. P.; Kaner, R. B.; Hoek, E. M. V. J. Mater. Chem. 2010, 20, 4621- 4628.) Each permeability and rejection data point was calculated based on five separate measurements for different membrane samples cast on different days. Characteristic pore size of membranes was determined from:

where " * ^sv , r_s is the solute particle radius, and r_p is the membrane pore radius. K. RESULTS AND DISCUSSION

1. MORPHOLOGY AND CHEMICAL COMPOSITION OF OXIDIZED SWCNTS.

[00114] An SEM image of raw SWCNTs shows nanofibrillar morphology together with large aggregates of carbon (Figure 2a, inset). After purification with strong acids

(HNO₃/H₂SO₄), the oxidized SWCNTs exhibit clear nanobundles with diameters between 5 and 20 nm, as shown in Figure 2a. An ATR/FT-IR spectrum of raw SWCNTs displays two bands near 1590 and 1250 cnf ¹ that can be attributed to the C=C stretching of the polyaromatic backbone of the nanotubes (Figure 3a). The oxidized SWCNTs exhibit an additional band near 1730 cnf ¹ (Figure 3b), indicating the formation of carboxylic acid functionalities on the sidewalls of the SWCNTs. (MacKenzie, K. J.; Dunens, O. M.; Hanus, M. J.; Harris, A. T. CarbonWll, 49, 4179-4190.) The presence of carboxylic acid groups offers opportunities for further derivatization reactions as well as better processability. The oxidization enables SWCNTs to form well-dispersed electrostatically stabilized colloids in water and ethanol as reported previously. (Shaffer, M. S. P.; Fan, X.; Windle, A. H.

Carbonl99S, 36, 1603-1612. Hu, L. B.; Hecht, D. S.; Griiner, G. Chem. Rev.2010, 110, 5790-5844.)

2. MORPHOLOGY, CHEMICAL COMPOSITION AND FLASH WELDING OF

SWCNTs/PANi COMPOSITES.

[00115] Carboxylic groups on the nanotubes can bind aniline monomer through hydrogen bonds and/or ionic bonds. Such specific interactions have proven to be effective in improving the miscibility of traditional polymer composites, and similar success is expected for SWCNT/PANi composites. Both SEM and TEM images of SWCNT/PANi composites show unique core-shell (SWCNT-PANi) ID nanostructures with average diameters of 60 nm, where oxidized SWCNTs were uniformly sheathed by 10-30 nm thick PANi coatings

(Figure 2b^~d). The composite dispersions with a 10% weight fraction have been created by dispersing the core-shell structured SWCNT-templated PANi EB nanofibers in NMP. By adopting the NIPS technique, ultrafiltration membranes processed from such dispersions exhibit smooth surfaces with an average pore diameter of 60 nm (Figure 4a). It appears that the core-shell ID nanostructures of SWCNT/PANi composites survived the NIPS. However, the surface coatings of PANi became smooth since part of the dedoped PANi may have been dissolved by the solvents. The surviving SWCNT-templated PANi nanofibers enable the membranes created to be welded by flash.

[00116] After exposure of the composite membrane to flash welding at full power, a large contrast can be readily seen between the welded (uncovered) and unwelded (covered) parts (Figure lb, top-right). The striking morphological effects of the composite membranes resulting from flash welding can be further imaged by SEM. The membranes show a distinct change in surface roughness, becoming smoother and shinier at sites exposed to the flash (Figure 4b). The cross-sectional SEM image reveals that welded sites increase from bottom (opposite to the flash exposure) to top (facing the flash exposure), leading to asymmetrically structured membranes (Figure 4c). Careful SEM analyses of the cross-section indicate that a large amount of oxidized SWCNTs were exposed (Figure 4d, arrows), in stark contrast to when all the nanotubes were sheathed by PANi before flash welding.

[00117] The SWCNTs survived the flash welding, indicating the carbon nanotubes have no changes on the flash welding of the PANi nanofibers. Therefore, combination of the SWCNTs and PANi nanofibers gives an ideal composite where one contributed to an excellent conductivity and another contributed a unique property of flash welding.

[00118] To obtain complete welding of the membrane, the flash should penetrate as much of the membrane as possible either by decreasing the thickness of the membrane or by increasing the intensity of the flash. Since the flash is impaired during penetration of the composite, this explains why asymmetric membranes are created. Upon adjusting the flash intensity from 6.25 to 12.5% (Figure 5a,b), the surface morphology of the composite membrane changes little until the power intensity is increased to 25% (Figure 5c). With increasing intensity, the membranes exhibit their smoothest and shiniest surface when 75% power of flash is applied (Figure 5d-f). The reason may be that low flash intensity is insufficient to weld the membranes, while excessive flash intensity causes uneven welding. In addition, upon increasing the number of flash welds, the pore size of the composite membranes increase (Figure 6), likely due to more welded sites.

[00119] Previous studies have shown a chemical cross-linking process occurs in PANi nanofibers when they are laser welded (Strong, V.; Wang, Y.; Patatanyan, A.; Whitten, P. G.; Spinks, G. M.; Wallace, G. G.; Kaner, R. B. Nano Lett.2011, 11, 3128-3135.) or heat treated. (Bhadra, S.; Khastgir, D. Polym. Degrad. Stab.2M%, 93, 1094-1099.) ATR/FT-IR spectroscopy is used to verify the occurrence of the chemical reactions generated by the flash welding, as displayed in Figure 7a,b. Unwelded SWCNTs/PANi composite membranes exhibit four characteristic bands at 1590, 1496, 1300 and 1 165 cnf ¹ that can be attributed to quinoid, benzenoid, C^~N aromatic amine and— N=quinoid=N— (electron-like band) stretching modes, respectively of PANi EB nanofibers (Liao, Y. Z.; Zhang, C; Zhang, Y.; Strong, V.; Tang, J. S.; Li, X. G.; Hoek, E. M. V.; Wang, K. L.; Kaner, R. B. NanoLett.2011, 11, 954-959.); an additional band at 827 cm^-1 can be assigned to the CA_T ^~H bending modes in ara-disubstituted aromatic rings (Mathew, R.; Mattes, B. R.; Espe, M. P. Synth. Met.2002, 131, 141-147.). Once the membranes are flash welded, the bands at 1590 and 1165 cnf ¹ blue-shift to 1602 and 1171 cm^-1, respectively, indicating that a chemical conversion of quinoid rings into benzenoid rings likely occurs during the flash welding process.

[00120] Table 1 shows the characteristic chemical shifts/transmittances of ATR/FT-IR spectral bands for SWCNT/PANi EB composite membranes under different numbers of flash welds (at full power). Moreover, the band at 1300 cnf ¹ red-shifts to 1282 cnf ¹ and the band at 827 cnf ¹ red-shifts to 81 1 cnf ¹ (Table 1), revealing that new bands such as tertiary amine nitrogen and trisubstituted benzene groups are generated by flash welding. Note that gas evolution can be observed during flash welding, probably due to dehydrogenation and/or addition reactions of CA_T ^~H groups with C^~N aromatic amine and quinoid groups (Figure 7c). Thus, we conclude that the "melted" appearance of the flash welded membranes is likely caused by a chemical cross-linking reaction. This is consistent with previous results of cross- linking processes induced by laser welding (Strong, V.; Wang, Y.; Patatanyan, A.; Whitten, P. G.; Spinks, G. M.; Wallace, G. G.; Kaner, R. B. Nano Lett.2011, 11, 3128-3135.) and heat treatment (Bhadra, S.; Khastgir, D. Polym. Degrad. Stab.2008, 93, 1094-1099.). With an increasing number of flash welds, the bands have no further chemical shifts, however, the percent transmittance increases (Table 1), indicating that the cross-linking density continues to increase. TABLE 1 raber of Baad I Band II Band ΙΠ Band !V Band V iks ill welds

(esr %) (cm'^! ¾) (enr^s %) (csr^s/%) (Cffi-^!/%)

0 1590/55.8 1496/46.1 1300/53.5 1165/54.4 827/48.1

1595/60.1 1496/30.3 1284/47.2 1167/55.6 812/44.9

1602/70.1 1496/31.2 1282/50.3 1 Π 1/63.0 811/47.6

3 1602/70.6 1 96/31.2 1282/52.3 1171/63.9 812/48.6

4 1602/71.4 1496/34.3 1282/53.3 1171/65.2 8 H/50.2

5 1602/75.6 1496/43.8 1282/59.5 1171/69.6 812/55.9

3. CONDUCTIVITY, PERMEABILITY AND REJECTION OF SWCNT/PANI COMPOSITE MEMBRANES.

[00121] We have shown that the morphology and chemical composition of the

SWNCT/PANi composite membranes can be readily changed by controlling the flash welding process. This technique can thus be utilized to tune the conductivity, permeability and rejection of SWCNT/PANi composite membranes. The influence of flash intensity and number of times flash welded on the conductivity of the SWCNT/PANi membranes is shown in Figure 8. Unwelded SWCNT/PANi composite membranes exhibit a sheet resistivity of ~5.6 ΜΩ/Τ as compared to > 100 ΜΩ Τ for PANi EB membranes. After increasing the exposure intensity from 25 to 50 to 75 and then to 100% of full power, the sheet resistivity decreases to 4.5, 3.5, 1.4 and 0.55 ΜΩ Τ, respectively. The improvements in conductivity of the composite membranes that result from flash welding at first appear to conflict with decreases in conductivity reported for pure PANi nanofibers. (Huang, J. X.; Kaner, R. B. Nat. Mater.2004, 3, 783-786. Strong, V.; Wang, Y.; Patatanyan, A.; Whitten, P. G.; Spinks, G. M.; Wallace, G. G.; Kaner, R. B. Nano Lett.2011, 11, 3128-3135.) However, here the PANi nanofibers used are dedoped and therefore electrically insulating. Since the dedoped PANi EB nanofibers don't contribute to the conductivity, the SWCNTs are primarily responsible for electronic transport within the composite membranes. As indicated by the analyses of morphology and chemical composition above, the sheathed SWCNTs become exposed due to "melting" of the PANi nano fibers. This actually leads to a lower percolation threshold for conductivity in the SWCNT/PANi composite membranes. Additionally, flash welding may reduce the oxidized SWCNTs in an effect similar to that observed with oxidized graphene (Cote, L. J.; Cruz-Silva, R.; Huang, J. X. J. Am. Chem. Soc. 2009, 131, 11027- 1 1032.) and laser-scribed graphene (M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science 2012, 335, 1326.), thus enhancing the conductivity of the SWCNTs. Both reasons help explain why the conductivities of the composite membranes increase after flash welding. Since the welding reaction occurs instantaneously, increasing the number of flash welds has a negligible effect on the chemical composition (Table 1). The SWCNT/PANi composite membranes therefore show stable conductive properties (Figure 8).

[00122] The influence of the number of flash welds on the permeability (i.e., pure water flux) and S1O2 and BSA nanoparticle rejections (i.e., selectivities) is shown in Figure 9. Unwelded SWCNT/PANi composite membranes reject 95% of Si0₂ and 28% of BSA nanoparticles. When the percent flash intensity is increased from 12.5 to 25, 50, 75 and 100%, the rejection of S1O2 is essentially unaffected (Figure 9a); however, the rejection of BSA decreases to 18.5, 14.5, 6.6, 2.2 and 1.4% (Figure 9b), respectively. These observations point to a strong dependence of membrane rejection on the type of the foulants involved, or more specifically, on their size. This implies that the asymmetric membranes exhibit excellent size-selective rejection properties, enabling particle size selective separations. Unwelded membranes display 54.7 gfd/psi of water flux. This value increases significantly to 208 and then to 266, 381, 398 and 415 gfd/psi when the percent flash intensity is increased from 12.5 to 25, 50, 75 and 100%, respectively (Figure 9c). The changes in surface roughness, pore size, porosity as well as bulk morphology upon the flash welding appear to explain the improvements in the water flux.

[00123] Note that, relatively high rejection is maintained for the larger S1O2 nanoparticles (48 nm), but low rejection is observed for the smaller size of BSA nanoparticles (6 nm) in both unwelded and welded membranes, classifying them as "loose" UF membranes. From the BSA rejection data we estimate the characteristic "average pore diameter" of these membranes to be 19, 24, 28, 42, 80 and 112 nm for flash welding intensities of 0, 12.5, 25, 50, 75 and 100%, respectively. By adjusting the physicochemical features of ultrafiltration membranes and molecules to be selectively transported, including charge, chemical interactions and molecular size, that can be exploited to enhance transport selectivity. (Jirage, K. B.; Hulteen, J. C; Martin, C. R. Sciencel997, 278, 655-658. Comper, W. D.; Glasgow, E. F. Kidney Int.1995, 47, 1242-1251.) Considering that these SWCNT/PANi asymmetric membranes operate at neutral pH, we therefore believe that the rejection of the charged BSA molecules could be enhanced by applying an electrical current through the membranes.

L. CONCLUSIONS

[00124] We have successfully synthesized SWCNT-templated PANi nanofibers by in situ chemical oxidative polymerization of aniline in an oxidized SWCNT suspension. The ID composite nanofibers that are produced exhibit a unique core-shell (SWCNT-PANi) structure with an average diameter of 60 nm and thicknesses of 10 to 30 nm for the PANi coating layer. Good processability and conductivity of the composite nanofibers enable formation of SWCNT/PANi ultrafiltration membranes that are conductive at neutral pH using well-established nonsolvent induced phase separation. Flash welding can be used to tune the morphology, porosity, conductivity, permeability and nanoparticle selectivity of the

SWCNT/PANi asymmetric ultrafiltration membranes. Upon flash welding, both the conductivity and pure water permeability of the membranes improved by a factor of 10, while the S1O₂ nanoparticle selectivity remained constant and above 95%. However, the BSA selectivity decreased due to the increase in pore size from flash welding. Thus, phase inverted SWCNT/PANi asymmetric membranes offer new degrees of freedom for making novel ultrafiltration membranes and flash welding enables further fine-tuning high electrical conductivity, hydrophilicity, water permeability and particle selectivity. The properties found here are amenable to cross-flow electro-ultrafiltration where the filtering membranes function as electrodes.

[00125] We have prepared a new hybrid material by synthesizing polyaniline in the presence of functionalized carbon nanotubes; thereby producing a polyaniline encapsulated carbon-nanotube-bundle structure that is readily dispersed in water and polar solvents; this enables the potential to cast the material into thin films with micropores, mesopores or macropores such as may be of interest in membrane separations. The result is a membrane with tunable separation performance based on post-treatment of "flash welding" which is enabled by the presence of polyaniline in the carbon nanotube-polyaniline hybrid material. [00126] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS What is claimed is:

1. A method for making a semi-permeable filtration membrane, the method comprising the steps of:

(a) polymerizing aniline in the presence of oxidized carbon nanotubes, thereby

forming an oxidized carbon nanotube-templated polyaniline nanofiber composite;

(b) solution casting the composite onto a nonwoven support fabric, thereby providing a membrane; and

(c) flash welding at least a portion of a surface of the membrane.

2. The method of claim 1, further comprising the step of isolating the composite before solution casting.

3. The method of claim 1, wherein solution casting is nonsolvent induced phase separation.

4. The method of claim 1 , wherein the carbon nanotubes are single-walled.

5. The method of claim 1, wherein the membrane is cast onto a nonwoven support fabric.

6. A semi-permeable filtration membrane prepared by the method of any of claims 1-5.

7. A semi-permeable filtration membrane comprising an oxidized carbon nanotube- templated polyaniline nanofiber composite solution cast onto a nonwoven support fabric.

8. The membrane of claim 7, wherein the membrane has been subjected to flash welding.

9. The membrane of claim 7, wherein the membrane is produced by solution casting.

10. The membrane of claim 9, wherein is solution casting is nonsolvent induced phase

separation.

1 1. The membrane of claim 7, wherein the carbon nanotubes are single- walled.

12. A method for tuning flux and/or selectivity in a semi-permeable filtration membrane, the method comprising the step of flash welding at least a portion of a surface of a membrane comprising an oxidized carbon nanotube-templated polyaniline nanofiber composite, thereby changing the membrane flux and/or selectivity compared to the membrane before performing the flash welding step.

13. The method of claim 12, wherein flux increases and the selectivity is unchanged.

14. The method of claim 12, wherein flux increases and selectivity decreases.

15. The method of claim 12, wherein the membrane is produced by the method of any of claims 1-5.